This invention relates to position transducers, and more specifically, to a system and method for determination of error parameters for performing self-calibration and other functions in a position transducer system.
Various movement or position transducers for sensing linear, rotary or angular movement are currently available. These transducers are generally based on either optical systems, magnetic scales, inductive transducers, or capacitive transducers.
In general, a transducer may comprise a read head and a scale. In an example of a 2-phase system, the transducer outputs two signals S1 and S2 that vary sinusoidally as a function of the position of the read head relative to the scale along a measuring axis. In one common concept for transducers, the signals S1 and S2 are intended to be identical except for a quarter-wavelength phase difference between them. The transducer electronics use these two signals to derive the instantaneous position of the read head relative to the scale along the measuring axis.
Ideally, the signals S1 and S2 are perfect sinusoids with no DC offsets, have equal amplitudes, and are in exact quadrature (i.e., a quarter-wavelength out of phase relative to each other, also referred to as xe2x80x9corthogonalxe2x80x9d herein). In practice, the signals S1 and S2 have small DC offsets, their amplitudes are not equal, and they have some orthogonality error. In addition S1 and S2 may have distorting spatial harmonic components. In addition, the transducer electronics can introduce additional errors such as offset, gain, and non-linearity errors.
In many practical transducers, the dominant sources of errors are offsets, and amplitude mismatches and phase errors between the phases. These errors are shown in the following equations for the case of a 2-phase system.                               S          1                =                              C            1                    +                                    V              1                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                                          2                ⁢                                  xe2x80x83                                ⁢                π                            λ                        ⁢                          (                              x                -                                  φ                  1                                            )                                                          (        1        )                                          S          2                =                              C            2                    +                                    V              2                        ⁢            cos            ⁢                          xe2x80x83                        ⁢                                          2                ⁢                                  xe2x80x83                                ⁢                π                            λ                        ⁢                          (                              x                -                                  φ                  2                                            )                                                          (        2        )            
In the above equations, x is a position, and xcex is a wavelength to the transducer output. The terms C1 and C2 produce offset errors. The terms V1 and V2 (if V1xe2x89xa0V2) produce amplitude mismatch errors. The terms xcfx861 and xcfx862 (if xcfx861-xcfx862xe2x89xa00xc2x0) produce phase mismatch errors, that is, phase relationship errors.
Alternatively, some transducers utilize a 3-phase system. The equations for a 3-phase system are shown below.                               U          R                =                              C            0                    +                                    A              0                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                                          2                ⁢                                  xe2x80x83                                ⁢                π                            λ                        ⁢                          (                              x                -                                  φ                  0                                            )                                                          (        3        )                                          U          S                =                              C            0            xe2x80x2                    +                                    A              0              xe2x80x2                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                                          2                ⁢                                  xe2x80x83                                ⁢                π                            λ                        ⁢                          (                              x                +                                  λ                  3                                -                                  φ                  0                  xe2x80x2                                            )                                                          (        4        )                                          U          T                =                              C            0            xe2x80x3                    +                                    A              0              xe2x80x3                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                                          2                ⁢                                  xe2x80x83                                ⁢                π                            λ                        ⁢                          (                              x                -                                  λ                  3                                -                                  φ                  0                  xe2x80x3                                            )                                                          (        5        )            
In the above equations, the terms C0, C0xe2x80x2, and C0xe2x80x3 produce offset errors. The terms A0, A0xe2x80x2, and A0xe2x80x3 (if they are not identical) produce amplitude mismatch errors. The terms xcfx860, xcfx860xe2x80x2, and xcfx860xe2x80x3 (if not all equal) produce phase mismatch errors.
One method for addressing errors such as those shown above is to calibrate the transducer. Calibrating the transducer and compensating for these errors requires determining or comparing the DC signal offsets, the amplitudes of the fundamental signals, the phase error between the fundamental signals, and insuring that they are adjusted or compensated to be equal. For further error compensation, the amplitudes of the harmonic components must also be considered.
One commonly used prior transducer calibration method is the xe2x80x9cLissajousxe2x80x9d method. The Lissajous method typically comprises inputting two nominally orthogonal read head signals to an oscilloscope, to drive the vertical and horizontal axis of the oscilloscope. The read head is continually scanned relative to the scale to generate changing signals. The oscilloscope display is observed, and the read head is physically and electronically adjusted until the display indicates a xe2x80x9cperfectxe2x80x9d circle, centered at zero on both axes. Under this condition, the amplitude, orthogonality, and offset of the two signals are properly adjusted.
The Lissajous method assumes that the two signals are both perfect sinusoids. Typically, there is no adjustment for harmonic errors which can distort the circle, as it is hoped that these are made insignificant by fixed features of the transducer design and assembly. The Lissajous method is well known to those skilled in the art, and has been performed by sampling the two signals with computer-based data acquisition equipment. However, to use this method in the case of a 3-phase system, the signals must generally to be converted to orthogonal signals before processing. Many three-phase systems either lack such signals, or access to such signals is either inconvenient or costly, making the Lissajous method inappropriate for many 3-phase transducer systems and products.
Alternatively, it has also been common to accept any transducer errors due to imperfect amplitudes, orthogonality, harmonics, and offsets, and to use an external reference, such as a laser interferometer, to accurately correct position errors from the read head at the system level, at predetermined calibration positions relative to the scale.
Position transducers typically require initial factory calibration, and periodic calibration or certification thereafter. In both cases, there is a cost for the associated equipment and labor. When the transducer is located in a remote location, it is difficult to set up the external data acquisition equipment and/or accurate external reference required for calibration. As a result, the transducer often has to be transported to another site or shipped back to the factory for calibration. This results in long downtime and increased costs.
Even in cases where the transducer does not have to be transported for calibration, the special tools and increased time required to set up the external display and/or reference result in increased costs and downtime. Thus, calibration and recalibration is often minimized or avoided, in practice. Since most practical position transducers are sensitive to variations during production, installation, and use, measurement errors normally increase in the absence of calibration, and in the periods between recalibration.
One example of a self-calibration method for an inductive position sensor is illustrated in U.S. Pat. No. 5,742,921. However, the method of the ""921 patent is closely tied to the operation of an associated motor, teaches positioning the transducer in order to acquire the desired data, and emphasizes calibration based on a signal range which is of little or no importance in certain implementations.
Another exemplary self-calibrating position transducer system that better-addresses some of the issues outlined above is illustrated in U.S. Pat. No. 6,029,363, which is commonly assigned and hereby incorporated by reference in its entirety. The method described in the ""363 patent samples two orthogonal output signals of the transducer at a plurality of evenly spaced positions within the one scale period, using the transducer itself as a position reference. The method then determines calibration values for the DC signal offsets, the amplitudes and non-orthogonality of the fundamental signals, and the amplitudes of the signal harmonic components using Fourier analysis techniques. Finally, the method corrects the signals using the determined calibration values. In summary, the method of the ""363 patent determines the error parameters by: (1) measuring the output voltages of the transducer as a function of a known position, and (2) analyzing the data using Fourier series techniques to derive the offset, amplitude, and phase mismatch parameters.
The method of the ""363 patent is effective for addressing many of the calibration issues described above, including the ability to compensate spatial harmonic errors. However, the method also introduces inconvenient data acquisition complexity and other constraints which reduce the convenience of the method in many practical situations. The present invention is directed to an alternative method for determination of error parameters for performing self-calibration and other functions without an external position reference in a transducer.
Most prior art position transducer systems do not include any way to conveniently identify and correct the various errors described above, outside of a permanent factory calibration environment. Furthermore, prior art methods have resulted in system error corrections that are fixed at the time of calibration, with no convenient way to adjust the corrections for subsequent alignment changes or aging of the transducer components. The present invention is directed to a system and method for determination of error parameters for performing self-calibration and other functions without an external position reference in a transducer. In various embodiments, the systems and methods of this invention uses a min-max method that involves measuring the transducer output signals as a function of time and does not require knowledge or recording of known positions. Thus, special tools or instruments are not required to calibrate the position transducer. In addition, the method is simple and fast enough that calibration/re-calibration or a related self-test operation may be performed continuously as a background task while the position transducer is operating. Thus, the system can keep the position transducer continually calibrated without interfering with the transducer""s normal operation.
In one embodiment, the method of this invention samples the output signals at a relatively high rate. The read head is moved relative to the scale at a speed v, along with an initial acceleration a. It is not necessary to know either v or a, but it is preferred that they be lower than a predetermined maximum value. The scan may be continued over a distance of several wavelengths. In this manner, the output signals are measured at various times according to the sample timing (not particular or approximate positions, since initial position, velocity and acceleration are not typically determined), as the read head is scanned relative to the scale. To find the offset for each phase, the minimum and maximum voltages for that phase are found. If a scan over several wavelengths is used, average values are computed. The offset for each phase is equal to the average between the maximum and minimum voltages. The signal amplitudes are equal to the difference between the maximum and minimum voltages, divided by 2. By comparing the amplitude for each phase, the amplitude mismatch is found. The phase mismatch error is found by first removing the amplitude mismatch and offset errors from each phase, and then comparing the relative amplitudes of the two signals at a given point in relation to what they ought to be with no phase relation error.
By utilizing the minimum and maximum voltages of a given phase to perform the calibration method, the method has a number of advantages. One advantage is that the calculation method is straightforward from the data, and does not require any iteration or data adjustment. In addition, less data needs to be saved in a memory because the method only requires the data near the peaks of the output waveform. Furthermore, unlike certain prior art methods which are required to be performed on two signals, the method can be applied to any number of signals from any number of phases. In addition, since only the peak values of the output function are utilized, the method is not so sensitive to unexpected distortions or spatial harmonics of the expected output/position transfer function.
The described method provides a fast calibration method which requires a minimum amount of data, and which can be automatically run in a background mode without inhibiting normal transducer operation. One embodiment where such a method has a particular utility is in inductive transducers. In particular, in many practical inductive transducers, offset and amplitude errors are the dominant errors. Furthermore, inductive transducers are increasingly used in low-power handheld instruments with limited computation speed and memory. Thus, the systems and methods of this invention are extremely well-matched for application to such systems.
Regarding another use of the invention, it should be appreciated that under significant contamination conditions, the signals of most other types of transducers degrade catastrophically, preventing erroneous use. However, inductive transducers are inherently designed to be able to continue functioning in the presence of contaminants. Thus, users may become complacent regarding the contamination of such transducers. In such cases, the self-calibration operations described herein are capable of detecting subtle changes in the operating signals of the inductive transducers, and they therefore provide a basis for alerting the user that contamination, or some other subtle accuracy problem, may be present. Of course, it should be appreciated that the systems and methods of this invention have this same utility in other types of transducers, when the contamination subtly degrades their signals, but is not serious enough to cause the previously described catastrophic signal degradations. Thus, the systems and methods of this invention are particularly important for identifying and correcting subtle operating problems in a wide variety of transducer systems with spatially periodic signals.
Another implementation where the systems and methods of this invention are particularly advantageous is one where increased accuracy is required. For example, it may be desired that a relatively low cost linear scale transducer produce measurements with 0.5 micron accuracy, for which calibration is essential.